ME 4447/6405

1. ME 4447/6405 Microprocessor Control of Manufacturing Systems and Introduction to Mechatronics Sensors Optical Encoder: Ryder Winck Laser Interferometer: Aaron Scott LVDT: Alexandre Lenoble

2. Optical Encoders Introduction Optical Encoder Components Types of Optical Encoders Encoder Discs and Digital Codes Encoder Reliability and Errors Applications Laser Interferometer What is a Laser Interferometer Types of Laser Interferometer How Do they Work Resolutions and Sampling Rate Applications Linear Variable Displacement Transducer (LVDT) What is a LVDT Types of LVDTs How Do they Work Resolutions and Sampling Rate Applications Ryder Winck Presentation Outline

3. Any transducer that changes a signal into a coded (digital signal) Optical Encoders Use light & photosensors to produce digital code (ie. Lab 3 encoder). Most popular type of encoder. Can be linear or rotary. Ryder Winck What is an Encoder?

4. 2 types of Optical Encoders: 1. Incremental (Lab 3 encoder) Measure displacement relative to a reference point. 2. Absolute Measure absolute position. Advantages – A missed reading does not affect the next reading. Only needs power on when taking a reading. Disadvantages – More expensive/complex. Cost/complexity proportional to resolution/accuracy. Ryder Winck Types of Optical Encoders

5. Light source(s) LEDs or IR LEDs provide light source. Light is collimated using a lens to make the beams parallel. Photosensor(s) Either Photodiode or Phototransistor. Opaque disk (Code Disk) One or more “tracks” with slits to allow light to pass through. Ryder Winck Fundamental Components

6. Ryder Winck Optical Encoder Components

7. Stationary “masking” disk Identical track(s) to Code Disk Eliminates error due to the diameter of the light beam being greater than the code disk window length. Signal amplifiers and pulse shape circuitry. Ryder Winck Other Components

8. Ryder Winck Quadrature • Two tracks (A & B) at 90 degrees offset. • Provide direction information. • Provides up to 4 times resolution.

9. Ryder Winck Encoder Disks Incremental Disk Absolute Disks Binary Gray Code

10. Example: 3 bit binary code Bit 0 Bit 1 Bit 2 Bit 0 Bit 1 Bit 2 Ryder Winck Absolute Disk Codes

11. Ryder Winck Problem with Binary Code • One angle shift results in multiple bit changes. • Example: 1 => 2 • 001 (start at 1) • 000 (turn off bit 0) • 010 (turn on bit 1)

12. Ryder Winck Problem with Binary Code • One degree shift results in multiple bit changes. • Example: 1 => 2 • 001 (start at 1) • 000 (turn off bit 0) • 010 (turn on bit 1) • It looks like we went from 1 => 0 => 2

13. One bit change per angle change. Bit 0 Bit 1 Bit 2 Bit 0 Bit 1 Bit 2 Ryder Winck Gray Code

14. Copy MSB. If MSB is 1, write 1s until next 1 is met. If MSB is 0, write 0s until next 1 is met. When 1 is met, logically switch what you are writing (1=>0 or 0=>1). Continue writing the same logical until next 1 is met. Loop back to step 3. Ryder Winck Converting from Gray Code to Binary Code

15. Copy MSB: 0_ _ _ Write 0s until next 1 is met: 00_ _ Switch to writing 1s: 001_ Write 1s: 0011 Ryder Winck Example: Convert 0010 to Binary Code

16. Copy MSB: 1_ _ _ Write 1s until next 1 is met: 1_ _ _ Switch to writing 0s until next 1 is met: 10_ _ Switch to writing 1s until next 1 is met: 1011 Ryder Winck Example: Convert 1110 to Binary Code

17. Resolution Incremental where N=# of windows. Resolution can be increased by reading both rising and falling edges ( ) and by using quadrature ( ). Absolute where n=# of tracks. Ryder Winck Encoder Reliability and Errors

18. Encoder errors Quantization Error – Dependent on digital word size. Assembly Error – Dependent on eccentricity of rotation (is track center of rotation=center of rotation of disk) Manufacturing tolerances – Code printing accuracy, sensor position, and irregularities in signal generation. Ryder Winck Encoder Reliability and Errors

19. Comment on pulse irregularity It is a result of noise in signal generation, variations in light intensity, and imperfect edges. It can be mitigated using a Schmidt Trigger, but this can lead to hysteresis. Using 2 adjacent sensor will negate this problem. Ryder Winck Encoder Reliability and Errors

20. More encoder errors Structural Limitations – Disk Deformation, physical loads on shaft. Coupling Error – Gear backlash, belt slippage, etc… Ambient Effects – Vibration, temperature, light noise, humidity, etc… Ryder Winck Encoder Reliability and Errors

21. Any linear/rotary position/velocity sensing DC Motor control – robotics/automation Mechanical computer mouse Digital readouts for measurement gauges Tachometers – planes, trains and automobiles Ryder Winck Applications

22. http://hades.mech.northwestern.edu/wiki/index.php/Image:Maxon-small2.jpghttp://hades.mech.northwestern.edu/wiki/index.php/Image:Maxon-small2.jpg http://www.designworldonline.com/Uploads/Leadership/Encoder_Montage1.jpg http://www.gpi-encoders.com/06_Technical_Articles.htm http://books.google.com/books?id=CjB2ygeR95cC&pg=PA630&lpg=PA630&dq=optical+encoder+mechatronics&source=bl&ots=uPB9nyu0AP&sig=PJYTMIG1dJ6UOPzj6uNhvYx1xSE&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPA639,M1 http://books.google.com/books?id=gUbQ9_weg88C&pg=PA97&lpg=PA97&dq=optical+encoders&source=web&ots=X2AbRCs5bL&sig=d-otsCBPIq7KGQodesPx3QJ_qos&hl=en&sa=X&oi=book_result&resnum=3&ct=result#PPA98,M1 http://books.google.com/books?id=uG7aqgal65YC&pg=RA1-PA163&lpg=RA1-PA163&dq=optical+encoders&source=web&ots=6-NhfhYb-F&sig=uf-VtBwSPRNUaCfujxu0gFb-xqY&hl=en&sa=X&oi=book_result&resnum=5&ct=result#PRA1-PA163,M1 http://mechatronics.mech.northwestern.edu/design_ref/sensors/encoders.html http://books.google.com/books?id=9e4Omibz3L4C&pg=PA395&lpg=PA395&dq=optical+encoders&source=web&ots=5bTXzKDiWG&sig=cGa9IdHuxw3Zq49SyVCJbzjGQnc&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA410,M1 Ryder Winck References

23. What is a Laser Interferometer? Types of Laser Interferometers How Do they Work? Resolutions and Sampling Rate Applications Aaron Scott Laser Interferometers

24. Interferometry = “interference” + “measurement” Basic application: hi-res measurement of distances Basic principle: superposition of light waves Constructive interference Destructive interference Aaron Scott What is a Laser Interferometer?

25. The Michelson Interferometer Aaron Scott What is a Laser Interferometer? • Difference in path length results in phase difference • Phase difference causes interference • Interference determined by analysis of fringe patterns

26. Brief historical background First American Nobel Prize in Sciences 1907 Optical precision instruments Invented the interferometer Most accurate measurement of c in his time Disproved existence of ether with famous Michelson-Morley experiment Aaron Scott What is a Laser Interferometer? Albert Michelson

27. Why “lasers” ? High coherence Collimated Predictable Frequency known Aaron Scott What is a Laser Interferometer?

28. Homodyne detection (standard interferometry) DC output signal from photodiode related to intensity of light from interference Both beams have same frequency Heterodyne detection One beam is frequency modulated prior to detection AC output signal of interference at the beat frequency (see board) Phase determined by signal analysis Aaron Scott Types of Laser Interferometers

29. Advantages of Heterodyne Detection AC signal frequency can be greatly reduced AC frequency = fbeat = fmod – fsignal Detection at low frequency reduces effect of high frequency noise Insensitive to ambient light and signal intensity Aaron Scott Types of Laser Interferometers

30. Homodyne – already discussed (Michelson interferometer) Heterodyne Dual frequency, polarized laser source Polarizing beam splitter Aaron Scott How Do They Work?

31. Representative values Resolution 10 nm digital resolution sub-angstrom analog resolution achieved by “external interpolation” Angstrom, Å = 1  10-10 m Sampling Rate 20 MHz Aaron Scott Resolutions and Sampling Rate

32. Michelson used his interferometer to measure the rotation rate of the Earth Perimeter of his ring was 1.9 km Aaron Scott Applications

33. 3 axis ring laser gyro Many winds of optic fibers achieve 1 km path Sensitive enough to measure Earth’s rotation despite small size Aaron Scott Applications

34. Distance measurement Profilometer to measure nanoscale surface features Nanopatterning Lithography Precision machining calibration High-precision linear feedback encoder Velocity measurement Doppler shift along measurement path changes beat frequency Aaron Scott Applications

35. Other measurements made possible by re-arrangements of the light paths. We can measure angle straightness flatness parallelism Aaron Scott Applications

36. LIGO Laser Interferometer Gravitational-Wave Observatory Gravity waves, predicted by Gen. Relativity, could be detected by sensing changes in length in perpendicular directions Light bounces 75 times before returning to be combined Each arm 4 km Aaron Scott Applications

37. LISA Laser Interferometer Space Antenna NASA/ESA expected 2018-2020 Similar to LIGO but MUCH larger 5 gigameter arm length 3 interferometers in 1 Aaron Scott Applications

38. http://en.wikipedia.org/wiki/Interferometry http://en.wikipedia.org/wiki/Albert_Abraham_Michelson http://encarta.msn.com/encyclopedia_761555191/Albert_Michelson.html http://www.renishaw.com/UserFiles/acrobat/UKEnglish/GEN-NEW-0117.pdf http://www.ligo-la.caltech.edu/contents/overviewsci.htm http://lisa.nasa.gov/ http://www.maxvalue.co.th/download/Excel.PDF DVD: “Albert A. Michelson Laboratory, History and Heritage” Public Release, NAWCWD, China Lake Aaron Scott References

39. LVDT Alexandre Lenoble

40. Linear Variable Displacement Transducer - Electrical transformer used to measure linear displacement Alexandre Lenoble What is a LVDT ?

41. Primary coil and 2 symmetric secondary coils Coils are encapsulated in metal/Epoxy - Ferromagnetic core Alexandre Lenoble Construction Secondary #1 Primary Secondary #2 Lead wires Displacement Moveable core

42. - Distinction by : - Power supply : - DC - AC Type of armature : - Unguided - Captive (guided) - Spring-extended Alexandre Lenoble LVDT Types

43. Easy to install Signal conditioning easier (equipment part of LVDT) Can operate from dry cell batteries - High unit cost Alexandre Lenoble DC LVDTs

44. Small size Very accurate – Excellent resolution (0.1 µm) Can operate with a wide temperature range (-65° F to +221° F) (30°F to 120°F for DC) - Lower unit cost than DC LVDTs Alexandre Lenoble AC LVDTs

45. - Unguided armature : - DC : $485 - AC : $330 - Spring-extended armature - DC : $1359 - AC : $1156 Alexandre Lenoble Cost per unit

46. Simplest mechanical configuration, armature fits loosely on the bore of the LVDT, being attached to the moving point by a male thread. - Armature completely separable from the transducer body. Alexandre Lenoble Unguided armature

47. Well-suited for short-range (1 to 50mm), high speed applications (high-frequency vibration) Alexandre Lenoble Unguided armature : applications

48. - Both static and dynamic applications Armature restrained and guided by a low-friction assembly Alexandre Lenoble Captive (guided) armature

49. Advantages compared to unguided armature : - Better for longer working range (up to 500mm) - Preferred when misalignment may occur Alexandre Lenoble Captive (guided) armature

50. - Armature restrained and guided by a low-friction assembly (as for captive armature) - Internal spring to continuously push the armature to its fullest possible extension Alexandre Lenoble Spring-extended armature